1. Field of the Invention
The present invention relates to a dispersion compensation device capable of compensating chromatic dispersion caused by optical fibers in a multiple-wavelength light transmission system so as to reduce waveform distortions due to the chromatic dispersion.
2. Description of the Prior Art
Recent years have seen the wide use of wavelength multiplexing technology as a measure to increase the amount of transmission through optical fibers. In the wavelength multiplexing technology, N light sources each of which can emit light with a different wavelength are disposed and each light is modulated with a data signal having a bit rate B, so that the amount of transmission is increased up to N*B. Recently, by using the wavelength multiplexing technology and light amplification technology, a system capable of transmitting lightwave signals with an amount of transmission of more than 100 Gbits at distances of up to 10,000 km is proceeding toward practical utilization.
Conventionally, in a 1.55 .mu.m-wavelength-range optical fiber transmission system, in order to reduce the occurrence of waveform distortions due to the chromatic dispersion caused by optical fibers to a minimum, dispersion shifted fibers or DSFs, which are designed so that the chromatic dispersion is zero at a wavelength of 1.55 .mu.m, have been used. If such prior art dispersion shifted fibers are used for a multiple-wavelength light transmission system, as previously mentioned, with a large amount of transmission, the system can be brought into a state in which the propagation constants for wavelengths which are next to each other are of almost equal, that is, phase matching is established, so that a unnecessary signal is generated due to the four-wave mixing. Although a description of the four-wave mixing will be omitted hereafter because an explanation for the four-wave mixing is given in detail by for example Govind P. Agrawal, "Nonlinear Fiber Optics", Academic Press, 1989, it should be noted that the four-wave mixing causes serious degradation of the transmission characteristic.
In order to prevent the four-wave mixing, nonzero dispersion shifted fibers whose zero dispersion wavelength is forced to deviate from a lightwave signal wavelength range are now going to use in multiple-wavelength light transmission systems. For example, in a case where multiple-wavelength light with a wavelength range of 1,550 nm to 1,560 nm is transmitted via a nonzero dispersion shifted fiber whose zero dispersion wavelength is 1,580 nm and whose dispersion slope is 0.1 ps/nm.sup.2 /km, a light component with a wavelength of 1,550 nm is transmitted while it undergoes -3 ps/nm/km of dispersion and a light component with a wavelength of 1,560 nm is transmitted while it undergoes -2 ps/nm/km of dispersion. Therefore, in this case, phase matching is not established adequately and hence this makes it difficult for the four-wave mixing to occur.
When such multiple-wavelength light is transmitted over great distances by way of a nonzero dispersion shifter fiber, each light component undergoes a cumulative, negative amount of dispersion. To reduce the average of the amounts of dispersion which all light components undergo to zero, so-called "dispersion management" is carried out. To do dispersion management, some 1.3 .mu.m zero-dispersion fibers of an appropriate length, each of which provides a certain amount of dispersion of +17 ps/nm/km for light with a wavelength of 1.55 .mu.m, are inserted at some midpoints in the transmission path. However, depending on the wavelength, a light component has an amount of residual dispersion, which has not been canceled, due to the gradient of the dispersion characteristic of nonzero dispersion shifted fibers, i.e. dispersion slope. For example, in a nonzero dispersion shifted fiber 10,000 km long in which dispersion management is carried out such that the dispersion for light with a wavelength of 1,555 nm is zero, a light component with a wavelength of 1,550 nm has a certain amount of residual dispersion of (1,555-1,550)*0.1*10,000 km = -5,000 ps/nm and a light component with, a wavelength of 1,560 nm has a certain amount of residual dispersion of (1,560-1,555)*0.1*10,000 km=+5000 ps/nm.
A sending terminal station performs a dispersion compensation operation on each light component with a certain wavelength in multiple-wavelength light including light components each having such residual dispersion, using a dispersion compensation fiber. Dispersion compensation that is carried out at a sending terminal station is called pre dispersion compensation. On the other hand, dispersion compensation that is carried out at a receiving terminal station is called post dispersion compensation. Sharing a needed amount of dispersion compensation half and half between the sending terminal station and the receiving terminal station is effective in compensating the residual dispersion in each light component, as disclosed by M. I. Hayee et al., "Pre-and Post compensation of dispersion and nonlinearities in 10-Gb/s WDM systems", IEEE Photonics Technology Letters, Vol. 9, No. 9, pp. 1271, 1997.
Referring now to FIG. 9, there is illustrated a block diagram showing the structure of an example of a wavelength multiplexing sending terminal station including eight optical sources and a plurality of prior art dispersion compensation devices each for performing pre dispersion compensation. In the figure, reference numerals 100a to 100h denote optical sources (or optical senders), i.e. OSs for sending out lightwave signals with wavelengths of .lambda..sub.-4, .lambda..sub.-3, .lambda..sub.-2, .lambda..sub.-1, .lambda..sub.+1, .lambda..sub.+2, .lambda..sub.+3, .lambda..sub.+4, respectively, 102a to 102g denote dispersion compensation fibers each for providing a positive amount of dispersion for light of a wavelength which lies in a 1.55 .mu.m wavelength range, 103a to 103g denote dispersion compensation fibers each for providing a negative amount of dispersion for light of a wavelength which lies in a 1.55 .mu.m wavelength range, 104a to 104i denote light amplifiers, and 105 denotes an optical multiplexer. Preferably, an array type waveguide grating or AWG is used as the optical multiplexer 105. In addition, reference numeral 106 denotes a transmission fiber in which dispersion management is carried out. A single mode fiber or SMF whose zero dispersion wavelength is typically 1.3 .mu.m is used as each of the plurality of dispersion compensation fibers 102a to 102g for providing a positive amount of dispersion. The amount of dispersion per a loss of 1 dB provided by one single mode fiber can be in the range of +80 ps/nm to +100 ps/nm. On the other hand, each of the plurality of dispersion compensation fibers 103a to 103g can produce a certain negative amount of dispersion of -240 ps/nm per a loss of 1 dB. In this specification, a dispersion compensation fiber for providing a negative amount of dispersion is simply referred to as a DCF. The reason why when comparing the path for a lightwave signal with a wavelength of .lambda..sub.+4 with the path for a lightwave signal with a wavelength of .lambda..sub.-4 in the system as shown in FIG. 9, for example, the number of light amplifiers disposed on the path for the lightwave signal with a wavelength of .lambda..sub.+4 is less than the number of light amplifiers disposed on the path for the lightwave signal with a wavelength of .lambda..sub.-4 is that the amount of dispersion per a loss of 1 dB caused by one DCF is greater than that caused by one SMF, that is, the dispersion efficiency of one DCF is greater than that of one SMF.
A description will be made as to the operation of the sending terminal station. The sending terminal station can provide a certain amount of dispersion for each lightwave signal of a certain wavelength using the plurality of dispersion compensation fibers 102a to 102g and the plurality of dispersion compensation fibers 103a to 103g, in consideration of the residual dispersion to be caused by the transmission fiber 106. Referring next to FIG. 10, there is illustrated a graph showing an example of the residual dispersion caused by the transmission fiber 106. In the figure, .lambda..sub.-i (i=1 to 4) denotes a wavelength which is shorter than the zero dispersion wavelength of the transmission fiber 106, and .lambda..sub.+i (i=1 to 4) denotes a wavelength which is longer than the zero dispersion wavelength of the transmission fiber 106. As shown in FIG. 10, since acumulative amount of dispersion which the lightwave signal with a wavelength of .lambda..sub.-i undergoes in the transmission fiber 106 is negative, a dispersion compensation using at least a fiber for providing a positive amount of dispersion is needed. On the other hand, since a cumulative amount of dispersion which the lightwave signal with a wavelength of .lambda..sub.+i undergoes in the transmission fiber 106 is positive, a dispersion compensation using at least a fiber for providing a negative amount of dispersion is needed. When the zero dispersion wavelength of the transmission fiber sits right at the center of the wavelength range of the multiple-wavelength light passing through the transmission fiber 106, as shown in FIG. 10, both the lightwave signal with a wavelength of .lambda..sub.-i and the lightwave signal with a wavelength of .lambda..sub.+i undergo cumulative dispersion of the same absolute,amount but of opposite sign in the transmission fiber 106, respectively.
When sharing a needed amount of dispersion compensation half and half between the sending terminal station and the receiving terminal station, the lightwave signal with a wavelength of .lambda..sub.-4 needs one-half of +7,000 ps/nm, i.e. +3,500 ps/nm of dispersion compensation, as can be seen from FIG. 10. Similarly, the lightwave signal with a wavelength of .lambda..sub.-3 needs +2,500 ps/nm of dispersion compensation, the lightwave signal with a wavelength of .lambda..sub.-2 needs +1,500 ps/nm of dispersion compensation, and the lightwave signal with a wavelength of .lambda..sub.-1 needs +500 ps/nm of dispersion compensation. It is clear from the summation of the amounts of dispersion compensation needed for all the lightwave signals that the plurality of positive dispersion compensation fibers alone need to perform +8,000 ps/nm of dispersion compensation. To do so, SMFs having a total length of more than about 470 km are needed and a large amount of space for mounting those fibers is therefore needed. On the other hand, the plurality of negative dispersion compensation fibers have to have a total length corresponding to -8,000 ps/nm of dispersion compensation. The needed total length corresponds to ten 16 cm-diameter reels.
Recently, a dispersion compensation device using a chirped grating fiber, which will be hereafter referred to as a dispersion compensation grating or DCG, has been developed specifically to solve the above problem of increasing the space for mounting a plurality of positive and negative dispersion compensation fibers. Referring next to FIG. 11, there is illustrated a block diagram showing the structure of such a prior art dispersion compensation device. In the figure, reference numeral 120 denotes a DCG, 121 denotes an input terminal, 122 denotes an output terminal, 123 denotes an optical circulator, 124 denotes a chirped grating fiber, and 125 denotes a reflectionless termination. A lightwave signal, which has been input to the DCG 120 by way of the input terminal 121, is furnished to the chirped grating fiber 124 by way of an intermediate terminal of the optical circulator 123. The chirped grating fiber 124 is a wavelength selectable reflector in which the grating pitch of the chirped grating fiber gradually increases along its length extending from one end portion connected to the optical circulator 123 to another end portion that is farther from the optical circulator 123, and has the reflection characteristic as shown in FIG. 12(a). As the wavelength of a light component included in the lightwave signal input via the input terminal decreases, it is reflected off a point in the chirped grating fiber 124 where is closer to the optical circulator 123 and is then furnished by way of the output terminal 122. On the contrary, as the wavelength of a light component included in the lightwave signal input via the input terminal increases, it is reflected off a point in the chirped grating fiber 124 where is farther from the optical circulator 123, and is then furnished by way of the output terminal 122. Thus, the chirped grating fiber 124 has the group delay characteristic as shown in FIG. 12(b). In other words, the chirped grating fiber 124 can serve as an optical circuit for providing a positive amount of dispersion of +1,000 ps/nm. Alternatively, the DCG 120 can be so constructed as to provide a negative amount of dispersion by making the grating pitch of the chirped grating fiber gradually decrease along its length extending from one end portion connected to the optical circulator 123 to another end portion that is farther from the optical circulator 123.
Referring next to FIG. 13, there is illustrated a block diagram showing the structure of an example of a wavelength multiplexing sending terminal station including eight optical sources and using a plurality of DCGs as shown in FIG. 11. In the figure, reference numerals 140a to 140d denote DCGs each for providing a positive amount of dispersion, and 141a to 141d denote DCGs each for providing a negative amount of dispersion. Thus, by using the plurality of DCGs as shown in FIG. 11 instead of the plurality of dispersion compensation fibers as shown in FIG. 9, the space needed for amounting the sending terminal station can be reduced largely.
However, a problem with the prior art dispersion compensation device or DCG, which is so constructed as mentioned above, is that a ripple in the amplitude of light and a variation in the group delay characteristic can cause a transmission penalty, a difficult manufacturing technique is needed to prevent a ripple in the amplitude of light and a variation in the group delay characteristic, and this results in increasing the manufacturing cost. Accordingly, the use of one dispersion compensation device or DCG for each wavelength in a transmission terminal station causes an increase in the cost of building the system.